In order to access oil and gas deposits located in underground formations it is necessary to drill bore holes into these underground formation and deploy production tubing to facilitate the extraction of the oil and gas deposits.
Additional tubing, in the form of well lining or well casing, may also deployed in locations where the underground formation is unstable and needs to held back to maintain the integrity of the oil/gas well.
During the formation and completion of an oil/gas well it is crucial to seal the annular space created between the casing and the surrounding formation. Also the annular space between the different sizes casings used as the well is completed. Additionally the annular space between the production tubing and said casing needs to be sealed. Further seals may be required between the underground formation and the additional tubing.
One of the most common approaches to sealing oil/gas wells is to pump cement into the annular spaces around the casing. The cement hardens to provide a seal which helps ensure that the casing provides the only access to the underground oil and gas deposits. This is crucial for both the efficient operation of the well and controlling any undesirable leakage from the well during or after the well is operated.
However it is not uncommon for crack/gaps (sometime referred to as micro annuli) to form in these cement seals over time, which lead to unwanted leakage from the well. One location where such cracks/gaps can form is at the interface between the production tubing and the cement seal.
In particular, when an oil/gas well is being operated in periodic, stop/start, manner the temperature within the production tubing can fluctuate significantly. These temperature fluctuations can cause the diameter of the production tubing to expand and contract. This movement applies pressure to the cement seal that can lead to the formation of small cracks/gaps in the seal, through which leakage can occur.
In order to address the formation of such crack/gaps in the cement seal it is known to deploy eutectic alloy, such as bismuth alloy, into the annular space and then heat the alloy to so that it melts and flows into the cracks/gaps. The alloy is then allowed to cool, wherein it expands to form an effective seal.
However there are disadvantages to this approach, not least because it requires at least a partial dismantling of the well so that the alloy can be deployed within the annular space, which can be time consuming and costly in terms of the down time of the well.
Another issue with this approach is ensuring that the alloy is delivered to the target region of the well in consistent and uniform manner so that the level of heat required to melt the alloy can be effectively pre-calculated, for example. This is important given that the process usually takes place deep underground and must be controlled remotely.